1. Field of the Invention
This invention relates to an improvement in a process for the production of xylose or xylitol wherein a vegetable material is contacted with a basic substance and the solid residue is thereafter treated with an acid. The improvement resides:
IN THE USE OF STONE NUT SHELLS,
IN EMPLOYING A TWO-STAGE DIGESTION INITIALLY WITH A 1.5 TO 6 MOLAR ALKALI METAL HYDROXIDE SOLUTION AT 10.degree. TO 50.degree. C,
thereafter diluting the digestion mass to 0.75 molar or less, and
DIGESTING AT A TEMPERATURE OF AT LEAST 60.degree. C.
2. Discussion of the Prior Art
D-(+)-xylose and its hydrogenation product, xylitol, are of not inconsiderable technical importance. Xylose can for example be used in the foodstuffs industry for various purposes, while xylitol has proved a very valuable sweetening agent for diabetics. As the starting material for the technical production of xylose there are used almost solely deciduous types of wood, such as beech and chestnut. The yields are about 10-12% (cf.e.g. German Patent Specification No. 912,440). A serious disadvantage of this process lies in that the residual wood substance, the "cellolignin," could not hitherto be put to any useful technical application, and that with this process only moderate xylose yields are obtained.
The obtaining of xylose from oat husks is already known from German Patent Specification No. 834,079. In this process the oat husks are heated with 0.08% ammonia to boiling point, or extracted with benzol -alcohol. Then follows the usual pressure hydrolosis with 0.2 to 0.5% H.sub.2 SO.sub.4 at 125.degree. C. No further processing takes place. In the preliminary treatment with NH.sub.3 4kg NH.sub.3 in an 0.08% solution are used for every 1,000 kg oat husks. However in order to separate off the acetic acid, 17 kg NH.sub.3 are necessary. Furthermore under the conditions mentioned in the German patent specification hardly any separation-off and hence elimination of the acetic acid, which makes up about 6% by weight of the oat husks, may take place.
Starting from this process there were proposed in German Offenlegungsschriften Nos. 2,358,407 and 2,358,472 processes for the production of xylose solutions by the digestion or pulping of deciduous wood or oat shells with a basic agent and treatment of the solid residue obtained with mineral acid. These processes which are characterised in that as the basic agent alkali hydroxide is used, in the first place permit the complete utilisation of the starting material and secondly give a higher xylose yield.
When using desiduous wood or oat husks as the starting material the applicant has found in both these cases that the alkali hydroxide solution used for the pulping or digesting in the first stage, only permitted a relatively low concentration. If for example a sodium hydroxide solution is used then its concentration amounts to no more than 4% by weight (= 1-molar solution) otherwise, with the necessarily high digestion temperatures, not only is the bound acetic acid removed but also too large a proportion of the lignin and the hemicelluloses passes into the solution. This means that the xylose yield diminishes with an increasing disintegration of the residue with diluted mineral acids.
It has now been found that when digesting or pulping hard shellnut type (hereinafter termed "stone nut") shells by the process of German Offenlegungsschrift Nos. 2,358,407 or 2,358,472, the maximum concentration indicated there of the alkali hydroxide solution of 4 percent by weight is not sufficient to effectively split off the bound acetic acid. A thorough-going removal of the bound acetic acid at the stage of the alkaline decomposition is therefore necessary, as the acetic acid would otherwise be split off with mineral acid in the second stage and would interfere with the neutralisation of the mineral acid, so that a considerable consumption of ion exchangers is necessary. Traces of acetic acid furthermore act deleteriously during the subsequent catalytic hydrogenation of xylose to xylitol as they poison the catalyst. In this case therefore a very careful cleaning of the xylose solutions is necessary.
When using an alkali hydroxide solution with a concentration of more than 4% by weight admittedly a more intense removal of the bound acetic acid can be anticipated in the first stage, but according to certain findings with deciduous wood or oat husks, the difficulties indicated above must be reckoned with.